Image sensor for sensing LED light with reduced flickering

ABSTRACT

An image sensor device has a first number of first pixels disposed in a substrate and a second number of second pixels disposed in the substrate. The first number is substantially equal to the second number. A light-blocking structure disposed over the first pixels and the second pixels. The light-blocking structure defines a plurality of first openings and second openings through which light can pass. The first openings are disposed over the first pixels. The second openings are disposed over the second pixels. The second openings are smaller than the first openings. A microcontroller is configured to turn on different ones of the second pixels at different points in time.

PRIORITY DATA

This application is a utility application of Provisional U.S.Application No. 62/735,886, filed Sep. 25, 2018, which is hereinincorporated by reference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. Among other applications,semiconductor ICs may be used to implement image sensors to senseradiation such as light. For example, complementarymetal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupleddevice (CCD) sensors are widely used in various applications such asdigital still camera or mobile phone camera applications. These devicesutilize an array of pixels (which may include photodiodes andtransistors) in a substrate to absorb (i.e., sense) radiation that isprojected toward the substrate and convert the sensed radiation intoelectrical signals.

However, conventional semiconductor image sensor devices may still havevarious shortcomings. For example, image sensor devices have pixels thatare selectively turned on and off for repeating cycles, where the pixelsare configured to collect light when they are turned “on” but not whenthey are turned “off”. While this type of operation is fine in mostsituations, it may present a problem with respect to light sources thatalso have a pulsing nature. For example, light-emitting diode (LED)devices may have “on” and “off” periods within each pulse cycle. The LEDdevices may emit light during the “on” period but does not emit lightduring the “off” period. As such, if the pulse frequency of the imagesensor device is not synched with the pulse frequency of the LED device,the image sensor device may capture “flicking” images of the LED device.In other words, the light from the LED appears as though it is“flickering” to the image sensor device, even though the human eye maystill observe a steady or continuously turned-on LED. When theflickering effect is produced, it is not only visually displeasing butcould also be dangerous, for example in automotive applications whereimage sensors are used to monitor a vehicle's surroundings, such astraffic signals or other signs that use LED light sources.

Therefore, while existing semiconductor image sensors have beengenerally adequate for their intended purposes, they have not beenentirely satisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion. It is also emphasized that thedrawings appended illustrate only typical embodiments of this inventionand are therefore not to be considered limiting in scope, for theinvention may apply equally well to other embodiments.

FIG. 1 includes images that illustrate a flickering effect.

FIG. 2 illustrates a diagrammatic fragmentary cross-sectional view of animage sensor device according to embodiments of the present disclosure.

FIG. 3 illustrates a diagrammatic fragmentary top view of an imagesensor device according to embodiments of the present disclosure.

FIGS. 4-6 illustrate diagrammatic fragmentary cross-sectional side viewsof an image sensor device according to embodiments of the presentdisclosure.

FIG. 7 illustrates a block diagram of a portion of an image sensordevice according to embodiments of the present disclosure.

FIG. 8 illustrates a timing diagram illustrating an operation of animage sensor device according to embodiments of the present disclosure.

FIG. 9 is a flowchart illustrating a method of operating an image sensordevice according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Still further, when a number or a range of numbers is described with“about,” “approximate,” and the like, the term is intended to encompassnumbers that are within a reasonable range including the numberdescribed, such as within +/−10% of the number described or other valuesas understood by person skilled in the art. For example, the term “about5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm.

Rapid advancements in the semiconductor industry have led to theproliferation of semiconductor devices in many fields. As an example,semiconductor devices have been made into image sensors, which canreplace or supplement the functionality of mechanically-oriented opticaldevices. For example, a semiconductor image sensor device may include anarray or grid of pixels for sensing and recording an intensity ofradiation (such as light) directed toward the semiconductor image sensordevice. In various implementations, a semiconductor image sensor devicemay include a charge-coupled device (CCD), complementary metal oxidesemiconductor (CMOS) image sensor (CIS), an active-pixel sensor (APS),or a passive-pixel sensor, etc.

During an operation of a semiconductor image sensor device, each of itspixels may be turned “on” or “off” periodically. During the “on” period(also referred to as an integration period), the pixels collect lightthat is projected toward the image sensor device. The collected light isconverted into electrical signals. During the “off” period, the imagesensor device processes the electrical signals to generate a capturedimage. The “on” and “off” periods of the image sensor device may definean operating frequency of the image sensor device.

While this type of operation for image sensor devices generally workswell for most situations, it may not be optimal for sensing light thatmay also be pulsing. For example, a light-emitting diode (LED) may emitlight at a particular pulsing frequency, where the LED emits lightduring an “on” period but does not emit light during an “off” period.The operating frequency of an image sensor device may not be the same asa pulsing frequency of an LED whose emitted light is supposed to becaptured by the image sensor device. When this occurs, the “on” period(during which the image sensor device captures light) of the imagesensor device and the “on” period (during which the LED emits light) ofthe LED may not always be synchronized. In other words, there may betime periods in which the LED is turned on and is emitting light, butunfortunately the image sensor device may be turned off during thesetime periods. As a result, the light emitted by the LED is not alwayscaptured by the image sensor device, or is only intermittently capturedby the image sensor device. The imperfect capturing of the LED light bythe image sensor device may product a flickering effect.

For example, the flickering effect is visually illustrated in FIG. 1with reference to images 50 and 60. In the non-limiting example shown inFIG. 1, the image 50 includes an output of an LED device captured by animaging device. The imaging device may be a mechanically-based camera(e.g., one that does not employ image sensor devices to capture images),or it may be an image sensor device whose operating frequency happens tobe “in sync” with the pulsing frequency of the LED device. In any case,the image 50 illustrates a clearly captured output of the LED device,which in this case is the number “130” surrounded by a circle or a ring.It is understood that the number “130” and the ring surrounding it maybe comprised of a plurality of individual LEDs, which are selectivelyilluminated in order to form the collective output of the LED device.

In comparison, the image 60 may illustrate an output of the LED devicethat is captured by an image sensor device whose operating frequency isnot in sync with the pulsing frequency of the LED device. As such, theremay be cycles where the pixels of the image sensor device are turned off(i.e., not capturing light), even though the LED device is turned on andis emitting light. Conversely, there may also be cycles where the pixelsof the image sensor device are turned on (i.e., capturing light), butthe LED device is turned off and is not emitting light. As a result, theimage sensor device captures a partial or incomplete output of the LEDdevice, which makes it difficult to read or understand the intendedoutput of the LED device. This partial or incomplete output of the LEDdevice may also vary from time to time. In other words, the image sensordevice may capture a first incomplete output of the LED device at afirst point in time, but the image sensor device capture a secondincomplete output of the LED device at a second point in time. Theoverall result may appear as if the LED device is displaying a“flickering” output to the image sensor device, even though the LEDdevice's output appears constant or continuous to a human eye.

The flickering effect is not only visually displeasing, but it may alsobe problematic in real world situations such as in automotiveapplications. For example, a self-driving vehicle may rely on imagesensor devices to capture the images within and/or surrounding thevehicle. The sources of light captured by the image sensor devices mayinclude light generated by LEDs, for example light from traffic signalsand/or instrument panel displays. The flickering effect may render thecaptured image unusable, which means that the image sensor device mayneed to recapture the image. Or worse, the self-driving vehicle may makea wrong decision based on the sub-optimally captured image (e.g.,reading a wrong LED output number or misinterpreting a traffic signal),which could interfere with the intended operation and/or compromise thesafety of the vehicle.

To overcome the problems discussed above, the present disclosureimplements, on an image sensor device, “small” pixels that have a lowerquantum efficiency (QE) than the standard pixels. The lower QE allowsthese small pixels to have a longer integration time (i.e., when theyare turned on) without being overexposed. A microcontroller controls theon and off times of the small pixels and the standard pixels such that,when a first one of the small pixels is turned off, a second one of thesmall pixels is turned on, and vice versa. In this manner, at least oneof the small pixels is turned on at all times during an operation of theimage sensor device, which allows the light output from LEDs to becaptured at all times. In other words, even if the standard pixels“miss” capturing the output of the LED device at a given point in timedue to not being synched with the LED device, at least one of the smallpixels can still capture the light emitted by the LED at that givenpoint in time. Consequently, the image sensor device of the presentdisclosure can substantially reduce or eliminate the flickering effect.The various aspects of the present disclosure are now discussed below inmore detail with reference to FIGS. 1-9.

Referring now to FIG. 2, a fragmentary cross-sectional side view of animage sensor device 100A is illustrated. The image sensor device 100Aincludes a device substrate 110. In some embodiments, the devicesubstrate 110 contains a silicon material doped with a p-type dopantsuch as boron (for example a p-type substrate). Alternatively, thedevice substrate 110 could contain another suitable semiconductormaterial. For example, the device substrate 110 may include silicon thatis doped with an n-type dopant such as phosphorous or arsenic (an n-typesubstrate). The device substrate 110 could also contain other elementarysemiconductors such as germanium and diamond. The device substrate 110could optionally include a compound semiconductor and/or an alloysemiconductor. Further, the device substrate 110 could include anepitaxial layer (epi layer), may be strained for performanceenhancement, and may include a silicon-on-insulator (SOI) structure.

Radiation-sensing regions—for example, pixels 120, 121, and 122—areformed in the device substrate 110. The pixels 120-122 are configured tosense radiation (or radiation waves), such as an incident light 130,that is projected toward device substrate 110 from a side 140. Theincident light 130 would enter the device substrate 110 through the side140 and be detected by one or more of the pixels 120-122. The pixels120-122 each include a photodiode in the present embodiment. In otherembodiments, the pixels 120-122 may include components such as pinnedlayer photodiodes, photogates, reset transistors, source followertransistors, and transfer transistors. The gate structures 131-135 ofthese transistors are shown in FIG. 2 as non-limiting examples. The gatestructures 131-135 may each include a gate dielectric, a gate electrode,and gate spacers formed on sidewalls of the gate dielectric and the gateelectrode. In some embodiments, the gate structures 131-135 areconfigured to transfer the image data captured by the correspondingpixels 120-122 and 160 to an external circuitry. The doped regions ofthe pixels 120-122 may be formed by one or more implantation processesfrom a side 150 opposite the side 140. Alternatively, the doped regionsof the pixels 120-122 may also be formed by one or more diffusionprocesses.

The pixels 120-122 may serve as the primary light-sensors of the imagesensor device 100A and may also be referred to as “standard” pixels. Forthe sake of simplicity, three “standard” pixels 120-122 are illustratedin FIG. 2, but it is understood that any number of “standard” pixels maybe implemented in the device substrate 110.

In addition to the “standard” pixels such as the pixels 120-122, theimage sensor device 100A further includes a plurality of “small” pixelsthat are also configured to capture radiation such as the light 130. Forreasons of simplicity, an example one of the small pixels is shown inFIG. 2 as the pixel 160, which is also formed in the substrate 110, butit is understood that the image sensor device may include a plurality ofother pixels similar to the pixel 160. These other pixels (similar tothe pixel 160) may be interspersed throughout the substrate 110, forexample each “small” pixel similar to the pixel 160 may be located amonga plurality of other “standard” pixels such as the pixels 120-122. Atotal number of the “small” pixels may be substantially less than atotal number of the “standard” pixels. For example, for every “small”pixel, there may be two or more “standard” pixels.

In some embodiments, the formation of the pixel 160 is similar to theformation of the pixels 120-122. For example, the doped region of thepixel 160 may also be formed by one or more implantation or diffusionprocesses. However, the pixel 160 has a lower QE compared to the pixels120-122. The lower QE may be achieved in a variety of ways, such as byconfiguring the doping concentration level of the pixels 120-122 and 160differently, or by configuring the physical sizes/volumes of the pixels120-122 and 160 differently, or by allowing different amounts or typesof light to be captured by the pixels 120-122 and 160. The lower QEallows the pixel 160 to be turned on for a longer period without beingoverexposed, which facilitates the capturing of the LED light, asdiscussed in greater detail below.

Still referring to FIG. 2, a plurality of isolation structures such asisolation structures 180-184 are formed in the device substrate 110. Theisolation structures 180-184 provide electrical and optical isolationbetween the pixels 120-122 and 160. For example, due to the presence ofthe isolation structures 180-184, light that is intended to be capturedby any given pixel is less likely to propagate into aneighboring/adjacent pixel, thereby reducing optical cross-talk betweenthe pixels. The isolation structures 180-184 also prevent the pixels120-122 and 160 from being electrically shorted together. In someembodiments, the isolation structures 180-184 include deep isolationtrenches (DTIs), which may include an optically dark or opticallyreflective dielectric material.

An interconnect structure 200 is formed over the substrate 110 (and overthe pixels 120-122 and 160 formed therein) on the side 150. Theinterconnect structure 200 includes a plurality of patterned dielectriclayers and conductive layers that provide interconnections (e.g.,wiring) between the various doped features, circuitry, and input/outputof the image sensor device 100A. For example, the interconnect structure200 includes an interlayer dielectric (ILD) and contacts, vias and metallines that are electrically isolated by the ILD. The contacts, vias, andmetal lines may include conductive materials such as aluminum,aluminum/silicon/copper alloy, copper, copper alloy, titanium, titaniumnitride, tantalum, tantalum nitride, tungsten, polysilicon, metalsilicide, or combinations thereof.

A passivation layer 210 is disposed over the interconnect structure 200.The passivation layer 210 protects the components of the image sensordevice 100A from elements such as dust, moisture, etc. In someembodiments, the passivation layer 210 contains a dielectric material,such as silicon oxide, silicon nitride, silicon oxynitride, etc.

The image sensor device 100A also includes a substrate 220 that isbonded to the device substrate 110, for example through the passivationlayer 210 and the interconnect structure 200. In other words, theinterconnect structure 200 and the passivation layer 210 are locatedbetween the device substrate 110 and the substrate 220. The substrate220 may be bonded to the device substrate 110 by molecular forces—atechnique known as direct bonding or optical fusion bonding—or by otherbonding techniques known in the art, such as metal diffusion or anodicbonding.

In some embodiments, the substrate 220 is a carrier substrate that mayinclude a silicon material or a glass material. The carrier substratemay provide mechanical strength and support when the device substrate110 is thinned. For example, in embodiments where the image sensordevice 100A includes a back-side illuminated image sensor—where thesides 140 and 150 respectively correspond to the back side and the frontside—the device substrate 110 may undergo a thinning process from theside 140. The thinning process may include a mechanical grinding processand a chemical thinning process. A substantial amount of substratematerial may be first removed from the device substrate 110 during themechanical grinding process. Afterwards, the chemical thinning processmay apply an etching chemical to the side 140 of the device substrate110 to further thin the device substrate 110 to an acceptably smallthickness, which may be on the order of a few microns or less. Thethinned device substrate 110 allows the pixels formed therein to havebetter light detection performance (especially in darker environments)and improved electrical and optical isolation therebetween.

In some embodiments, the substrate 220 may also contain electricalcircuitry, for example electrical circuitry for an application-specificintegrated circuit (ASIC). The electrical circuitry may be electricallycoupled to the components of the device substrate 110 (e.g., to thepixels 120-122 and 160) through the interconnect structure 200 and/orother through-substrate vias/contacts. As such, the substrate 220 mayalso be referred to as an ASIC substrate. It is understood that the ASICsubstrate may also be capable of providing the mechanical strength andsupport during the thinning of the device substrate 110.

Still referring to FIG. 2, a passivation layer 250 is formed over thedevice substrate 110 on the side 140. The passivation layer 250 alsoprotects the components of the image sensor device 100A from elementssuch as dust, moisture, etc. Since the light 130 needs to propagatethrough the passivation layer 250 to reach the pixels 120-122 and 160,the passivation layer 250 is transparent. In some embodiments, thepassivation layer 250 contains a dielectric material, such as siliconoxide, silicon nitride, silicon oxynitride, etc.

A grid structure 270 is embedded in the passivation layer 250. Portions270A, 270B, 270C, 270D, and 270E of the grid structure 270 are locatedover the isolation structures 180-184, respectively. To facilitate theunderstanding of the grid structure 270, a fragmentary top view of aportion of the grid structure 270 is also shown in FIG. 3. For example,FIG. 3 illustrates the top views of the pixels 120-121 and 160 shown inFIG. 2, as well as neighboring pixels 123-128 that are similar to thepixels 120-121 (e.g., the “standard” pixels). The grid structure 270defines a plurality of openings, such as openings 280-282 and 290 thatare at least partially vertically aligned with the pixels 120-122 and160, respectively. In other words, each of the pixels 120-128 and 160 isat least partially exposed by a respective opening defined by the gridstructure 270. In some embodiments, the pixels 120-128 may be at leastpartially offset from the respective openings defined by the gridstructure 270 as well.

The grid structure 270 has a material composition that is opticallyopaque and/or reflective of radiation (e.g., the light 130). In someembodiments, the grid structure 270 has a metal material composition. Inother embodiments, the grid structure 270 has a dielectric materialcomposition that is optically opaque. Since the light 130 cannot passthrough the grid structure 270, the grid structure 270 can control theamount of light received by each of the pixels below by configuring thesize of the openings aligned with each pixel. For example, the opening280 (defined at least in part by the portions 270A and 270B of the gridstructure 270) exposes a substantial majority (or even an entirety) ofthe pixel 120, and thus the pixel 120 receives a first amount of light.

In comparison, the opening 290 (defined at least in part by the portions270B and 270C of the grid structure 270) exposes a small portion of thepixel 160, and thus the pixel 160 receives a second amount of light thatis substantially less than the first amount received by the pixel 120.In some embodiments, the amount of light received by each pixel may becorrelated with the size of the respective opening aligned with thepixel. For example, the opening 280 has a lateral dimension 300, and theopening 290 has a lateral dimension 310. According to embodiments of thepresent disclosure, the lateral dimension 300 is greater than thedimension 310, for example by at least five times. As such, the amountof light received by the pixel 120 is at least twice the amount of lightreceived by the pixel 160, which causes the pixel 160 to have a lower QEthan the pixel 120. As discussed above, the lower QE of the pixel 160allows it to be turned on for a longer period of time, which facilitatesthe capturing of LED light with a pulsing frequency.

Instead of (or in addition to) restricting the amount of light beingreceived by the pixel 160, the present disclosure also offers other waysto reduce the QE of the pixel 160. For example, the pixel 160 may beformed to have a different dopant/doping concentration level than thepixels 120-122. Since QE is determined at least in part by the dopingconcentration level of the pixel, the QE of the pixel 160 may beconfigured to be lower than the QE of the pixels 120-122, for example bylowering the doping concentration level of the pixel 160 in someembodiments, or raising the doping concentration level of the pixel 160in other embodiments.

Referring now to FIG. 4, another embodiment of the image sensor deviceof the present disclosure is illustrated as an image sensor device 100B,which also offers reduced QE for the pixel 160. For reasons ofconsistency and clarity, similar components appearing in both FIG. 2 andFIG. 4 are labeled the same. One difference between the image sensordevice 100A in FIG. 2 and the image sensor device 100B in FIG. 4 is thatthe pixel 160 in the image sensor device 100B is substantially smallerin size than the pixel 160 in the image sensor device 100A. For example,whereas the pixel 160 may have a similar size as the pixels 120-122 inthe image sensor device 100A, the pixel 160 in the image sensor device100B is substantially smaller in size than each of the pixels 120-122.As shown in FIG. 4, the pixels 120-122 may each have a lateral dimension330, whereas the pixel 160 may have a lateral dimension 340 that issubstantially smaller than the lateral dimension 330 (e.g., at least 50%smaller). In some embodiments, the smaller size of the pixel 160 can beachieved by configuring a mask used in the implantation process toimplant dopants into the substrate 110 in the pixel formation process.For example, the mask may have greater openings for the pixels 120-122but a smaller opening for the pixel 160, where dopants are implantedthrough the openings to define the size of the pixels 120-122 and 160.

The smaller size of the pixel 160 corresponds to a smaller amount oflight that can be collected by the pixel 160. As such, even if the gridstructure 270 does not specifically define a smaller opening 290 for thepixel 160, the smaller size of the pixel 160 means that it will stillhave a lower QE than the other pixels 120-122.

Another embodiment of the image sensor device is illustrated in FIG. 5as image sensor device 100C. The image sensor device 100C is similar toimage sensor device 100A and 100B and may be viewed as a combination ofthe image sensor device 100A and 100B. In more detail, instead of apixel 160 having a substantially uniform lateral dimension, the pixel160 in FIG. 5 has two (or more) portions, such as a portion 160A and aportion 160B that is narrower than the portion 160A. This type ofgeometric profile of the pixel 160 may be achieved by using two (ormore) implantation processes to form the pixel 160, for example usingone implantation process to form the portion 160A and using anotherimplantation process to form the portion 160B. The smaller portion 160Ameans that the pixel 160 still receives less light and therefore has alower QE than the pixels 120-122.

In addition, the image sensor device 100C implements multiple isolationstructures 181 and 185 between the pixel 120 and the pixel 160, as wellas multiple isolation structures 186 and 182 between the pixel 121 andthe pixel 160. The multiple isolation structures further reduces opticalcross-talk between the pixels and helps guide the light to be receivedby their intended pixels.

Yet another embodiment of the image sensor device is illustrated in FIG.6 as image sensor device 100D. As shown in FIG. 6, color filters such ascolor filters 400-402 and 410 are implemented in the image sensor device100D. The color filters 400-402 and 410 are formed over the passivationlayer 250 and are vertically aligned with the pixels 120-122 and 160,respectively. The color filters 400-402 and 410 may contain an organicmaterial and may be formed by one or more coating and lithographyprocesses.

The color filters 400-402 and 410 may also be associated with differentcolors. For example, the color filter 400 is a red color filter and mayallow a red light to pass through but will filter out all the othercolors of light. The color filter 401 is a blue color filter and mayallow a blue light to pass through but will filter out all the othercolors of light. The color filter 402 is a green color filter and mayallow a green light to pass through but will filter out all the othercolors of light. The color filter 410 may be a grey color filter in theillustrated embodiment, which may be more absorptive with respect tovarious spectra of light than the color filters 400-402. Note that thegrey color filter is merely a non-limiting example of a broader band(e.g., broader than just the red, blue, and green bands) non-RGB colorfilter, and in other embodiments, different types of broader band colorfilters may be implemented instead. Due to the differences between thecolor filter 410 and the color filters 400-402, the pixel 160 has alower QE than the pixels 120-122. In some embodiments, the color filter410 causes the pixel 160 to have a QE that is less than 5%.

FIG. 7 illustrates a simplified block diagram of a portion of the imagesensor device 100A/B/C/D of the present disclosure. The image sensordevice 100A/B/C/D includes a controller 500, for example amicrocontroller. The controller 500 may be implemented on the devicesubstrate 110 in some embodiments or may be implemented on the substrate220 (e.g., when the substrate includes an ASIC) in other embodiments.The controller 500 is electrically coupled to the pixels 120-122 and aplurality of other “standard” pixels similar to the pixels 120-122, suchas pixel M. The controller 500 is also electrically coupled to the pixel160 and a plurality of other “small” pixels similar to the pixel 160,such as pixel N. In some embodiments, the image sensor device 100A/B/C/Dmay include a first number of pixels 120 through M and a second numberof pixels 160 through N. In some embodiments, the first number issubstantially equal to the second number, meaning that there may beabout the same numbers of “standard” pixels and “small” pixels. In someother embodiments, the first number is greater than the second number.For example, the first number may be at least two times greater than thesecond number, meaning that there are many more of the “standard” pixelsthan the “small” pixels. In some other embodiments, the first number issmaller than the second number. For example, the first number may beabout ½ of the second number, meaning that there are fewer of the“standard” pixels than the “small” pixels.

As discussed above, the pixels 120 through M are pixels having a greaterQE, while the pixels 160 through N are pixels having a lower QE. Invarious embodiments, the lower QE may be achieved through a smaller gridopening to let less light pass through and reach the pixels 160 throughN, or by forming the pixels 160 through N to have smaller photosensitiveregions than the pixels 120 through M, or by configuring the colorfilter of the pixels 160 through N to be more absorptive to light (e.g.,grey color filters or other broader band color filters) than the colorfilters of the pixels 120 through M.

It is understood that the pixels 120 through M may be considered theprimary pixels for the image sensor device 100A/B/C/D, since most of theimage capturing is performed by these pixels, while the pixels 160through N may be considered the secondary pixels for the image sensordevice 100A/B/C/D, since they mostly facilitate in the image capturingoperation by capturing images when the pixels 120 through N cannot.

This is explained in more detail with reference to FIG. 8, whichillustrates a simplified example timing diagram 800. The timing diagram800 includes a pulse 810 of an LED device, a pulse 820 of a “standard”pixel (e.g., the pixels 120-122), a pulse 830 of a first small pixel(e.g., one of the pixels 160), and a pulse 840 of a second small pixel(e.g., another one of the pixels 160). The pulses 810-840 each have anX-axis component and a Y-axis component. The X-axis represents time,whereas the Y-axis represents magnitude or amplitude of each of thepulses 810-840.

As discussed above, LED devices typically emit light periodically andthus may be “pulsing”. This is shown in FIG. 8, where the pulse 810 ofthe LED has “on” periods 810A, 810C, 810E, and 810G, with the “off”periods 810B, 810D, and 810F disposed between the “on” periods. The LEDdevice emits light during the “on” periods but does not emit lightduring the “off” periods. Since the emission of light is periodic, itmay be said that the LED device has a light-emitting frequency, forexample a frequency of F1.

The pulse 820 of the “standard” pixels has “on” periods 820A, 820C, and820E (also referred to as integration periods) where the “standard”pixels detect light, as well as “off” periods 820B, 820D, and 820F wherethe “standard” pixels do not detect light. The turning on and off ofeach of the “standard” pixels is performed by the controller 500 of FIG.7 and has a frequency F2. It is understood that although the length ofthe “on” periods 820A/820C/820E does not appear to be substantiallydifferent from the length of the “off” periods 820B/820D/820F in FIG. 8,this may not be drawn to scale. In actual operation of the image sensordevice, the length of the “on” periods 820A/820C/820E may besubstantially shorter than the length of the “off” periods820B/820D/820F, for example multiple times shorter. This may be done toavoid overexposing the pixels, as well as to give the image sensordevice sufficient time to process the captured signals during the “off”periods.

The “standard” pixels can capture light emitted from the LED deviceperfectly if the frequencies F1 and F2 are the same and the pulses 810and 820 are phase-synched (e.g., having “on” periods at the same time).Unfortunately, this situation is rare and typically does not occur. Morerealistically, the frequencies F1 and F2 are not identical (or even ifthey are identical, the pulses 810 and 820 are not phase-synched). Forexample, as shown in FIG. 8, the pulse 810 has a greater frequency F1than the frequency F2 of the “standard” pixels, since the cycles of the“on” and “off” periods occur faster for the pulse 810. In this example,when the LED device is emitting light during the “on” period 810A, the“standard” pixels can adequately capture the emitted LED light, sincethe “standard” pixels are activated during the “on” period 820A as well,meaning the “on” periods 810A and 820A substantially overlap.

However, when the LED device is emitting light during the “on” period810C, the “standard” pixels do not fully capture the emitted LED light,since the “standard” pixels are activated (e.g., the beginning of the“on” period 820C) after much of the “on” period 810C has elapsed. Thesmall overlap between the “on” periods 810C and 820C could result in aninaccurate image capture of the LED device. The situation is even worsewhen the LED device is emitting light during the “on” periods 810E or810G, since the “standard” pixels are turned off (i.e., in the “off”periods 820D or 820F) at this time. Therefore, no LED image can becaptured during this period. The incomplete or inaccurate image captureof the LED device may result in the flickering effect discussed above.In addition, the relatively short “on” periods (compared to the “off”periods) of the “standard” pixels may exacerbate this problem, since thetime window for image capturing is short.

The present disclosure overcomes the flickering problem by implementingtwo or more “small” pixels (e.g., the pixel 160) having lower QE. Thelower QE allows the “small” pixels to stay turned on for a much longerperiod of time without getting over exposed. For example, as shown inFIG. 8, the first “small” pixel has an “on” period 830A and an “off”period “830B”, while the second “small” pixel has an “off” period 840Aand an “on” period “840B”. Compared to the “on” periods 820A, 820C, and820E of the “standard” pixels, the “on” periods “830A” and “840B” of the“small” pixels are substantially longer, for example several timeslonger. In some embodiments, the “on” periods “830A” and “840B” of the“small” pixels are about 2-100 times longer than the “on” periods of the“standard” pixels. The longer “on” periods of the “small” pixels allowthem to capture LED light when the “standard” pixels cannot. Forexample, when the LED device is emitting light during the “on” period810C, the first “small” pixel is also turned on during the period“830A”. As such, the first “small” pixel can capture the LED light eventhough the “Standard” pixels may have trouble doing so.

The controller 500 also selectively turns on and off the “small” pixelssuch that when the first “small” pixel is turned on, the second “small”pixel is turned off, and vice versa. Therefore, when the LED device isemitting light during the “on” period 810G, the second “small” pixel isalso turned on during the period “840B”, even though the “standard”pixels and the first “small” pixel are turned off during this time. Insome embodiments, the controller 500 ensures that at least one of the“small” pixels is turned on at all times during an operation of theimage sensor device. In this manner, at least one of the “small” pixelswill be able to capture the LED light at all times, even if all theother “standard” pixels and the other “small” pixels are turned off.

Based on the above, it can be seen that the “small” pixels of the imagesensor device are each selectively activated or turned on and off at afrequency F3 that is substantially lower (e.g., at least several timeslower) than the frequency F2 at which the “standard” pixels are eachturned on and off. In some embodiments, the frequency F2 is at leastseveral times faster or greater than the frequency F3. For example, thefrequency F2 may be about 2-100 times faster than the frequency F3. Theslower frequency F3 of the “small” pixels allows its “on” period to belonger so as to capture multiple pulses of the LED light. In someembodiments, the controller 500 may determine a frequency F1 of the LEDdevice, and based on the determination, it may set the frequency F3 forturning on and off the “small” pixels at a value that is less than ½ ofthe frequency F1. For example, if the LEDs have a frequency F1 that is100 Hz, then the controller 500 may set the frequency F3 of the “small”pixels to be less than 50 Hz (and could be much lower than 50 Hz inactual implementation). This allows each pulse of the “small” pixels tocapture two or more pulses of the LED device.

It is understood that although two “small” pixels are used herein toillustrate the concept of the present disclosure, three or more “small”pixels may be used in actual implementation of the image sensor deviceas well. This may be helpful when the “off” period of the “small” pixelsis substantially longer than the “on” periods of the “small” pixels. Inthese embodiments, a first “small” pixel may be turned on, followed by asecond “small” pixel, then a third “small” pixel, so on and so forth,until the first “small” pixel is turned on again.

It is also understood that even though FIG. 8 shows the first “small”pixel and the second “small” pixel as having exactly opposite “on” and“off” periods, it is not intended to be limiting. In other embodiments,the first “small” pixel and the second “small” pixel may also have atleast partially overlapping “on” periods and/or at least partiallyoverlapping “off” periods for reasons of redundancy.

It is further understood that although the present disclosure uses LEDlight as an example, the image sensor device discussed herein may beused to capture light from other types of devices that produce pulsinglight. In other words, whereas any device that emit light based on anon/off cycle may present flickering problems for conventional imagesensors, the image sensor of the present disclosure can accuratelycapture the images of these light-emitting devices with no or verylittle distortion.

For reasons of simplicity, not all of the fabrication processesassociated with the formation of the image sensor devices 100A-100D arediscussed in detail herein. However, it is understood that thefabrication of the image sensor devices 100A-100D may follow a backsideilluminated (BSI) image sensor flow. For example, the gate structures131-135 may be formed over the device substrate 110 by variouspatterning processes, the photosensitive regions such as pixels 120-122and 160 may be formed in the device substrate 110 through ionimplantation processes, and the interconnect structure 200 may be formedby a plurality of patterning/deposition/polishing processes on the“front” side 150 of the device substrate 110. The passivation layer 210may be formed on the interconnect structure 200. The “front” side 150 ofthe device substrate 110 is then bonded with the carrier substrate 220,such that the interconnect structure 200 and the passivation layer 210are located between the device substrate 110 and the carrier substrate220. A backside grinding process may then be performed from the “back”side 140 of the device substrate 110, where various mechanical and/orchemical processes are performed to “thin down” the device substrate 110to an acceptable thickness level, which may be in a range between about2 microns and about 100 microns. After the “thin down process” isperformed, the grid structure 270 and the color filters 400-402 and 410may be formed on the “back” side 140 of the device substrate 110.Although not shown herein for reasons of simplicity, micro-lenses mayalso be formed over the color filters 400-402 and 410. As non-limitingexamples, the details of the BSI fabrication process flow may be foundin U.S. Pat. Nos. 9,385,156, 8,736,006, 8,772,899, the disclosures ofeach of which are hereby incorporated by reference herein in theirrespective entireties.

FIG. 9 is a flowchart illustrating a method 900 of operating an imagesensor device according to an embodiment of the present disclosure. Themethod 900 includes a step 910, in which a plurality of first pixels isturned on and off at a first frequency. Each of the first pixels senseslight at a first quantum efficiency. The method 900 includes a step 920,in which a plurality of second pixels is turned on and off at a secondfrequency that is less than the first frequency. Each of the secondpixels senses light at a second quantum efficiency that is lower thanthe first quantum efficiency.

In some embodiments, the turning on and off the plurality of firstpixels and the turning on and off the plurality of second pixels areperformed to sense light emitted by one or more light-emitting diode(LED) devices.

In some embodiments, the first frequency is at least multiple timesgreater than the second frequency.

In some embodiments, the turning on and off the plurality of firstpixels and the turning on and off the plurality of second pixelscomprise turning on at least one of the second pixels while all of thefirst pixels are turned off.

In some embodiments, the plurality of second pixels includes a firstsecond pixel and a second second pixel, and wherein the turning on andoff the plurality of second pixels comprises: at a first period in time,turning on the first second pixel and turning off the second secondpixel; and at a second period in time different from the first period intime, turning off the first second pixel and turning on the secondsecond pixel. In some embodiments, the turning on and off the pluralityof second pixels comprises: turning on at least one of the second pixelsat any point in time during an operation of the image sensor device.

It is understood that additional processes may be performed before,during, or after the steps 910-920 of the method 900. For example, themethod 900 may include: determining a frequency of the one or more LEDdevices, and based on the determined frequency of the LED devices, thesecond frequency is set to be less than ½ of the frequency of the one ormore LED devices. Other additional steps are not discussed herein forreasons of simplicity.

In summary, the present disclosure implements “small” pixels on an imagesensor device that includes “standard” pixels. The “small” pixels havelower QE than the “standard” pixels. The smaller QE allows the “small”pixels to be turned on substantially longer (i.e., having longerintegration times) than the “standard” pixels. The controller of theimage sensor device also selectively turns on and off the “small” pixelsso as to make sure different ones of the “small” pixels are turned onand off at different points in time. For example, a first “small” pixelmay be turned on while a second “small” pixel is turned off, and thefirst “small” pixel may be turned off while a second “small” pixel isturned on. The frequency at which each “small” pixel is turned on andoff is substantially less than a frequency at which each “standard”pixel is turned on and off, as well as less than a frequency at which anLED device is turned on and off to emit light.

Based on the above discussions, it can be seen that the presentdisclosure offers advantages over conventional image sensor devices. Itis understood, however, that other embodiments may offer additionaladvantages, and not all advantages are necessarily disclosed herein, andthat no particular advantage is required for all embodiments. Oneadvantage is the reduction or elimination of the flickering effect. Inmore detail, the longer integration time (e.g., the “on” period) of thesmall pixels allows multiple pulses of LED light to be captured. Thearrangement in which a first “small” pixel is turned on while a second“small” pixel is turned off (and vice versa) ensures that at least oneof the “small” pixels is turned on at all times to capture the LEDlight. Thus, even during time periods where the “standard” pixel cannotcapture the LED light (due to the “standard” pixels being turned offwhen the LED is emitting light), the “small” pixels can still capturethe LED light. As such, the final image captured by the image sensordevice is accurate and will not exhibit the flickering effect. Inaddition to improving the visual appearance, the image sensor devicesherein also offers enhanced security and safety, particularly when theyare used in automotive applications, such as self-driving vehicles.Another advantage of the present application is that the implementationof the “small” pixels is simple and does not require significant changesto the current image sensor structure or the fabrication thereof. Insome embodiments, the present disclosure merely changes the layoutdesign of the grid structure to achieve the “small” pixels with lowerQE. In other embodiments, the lower QE of the “small” pixels may beachieved by configuring the implantation processes for forming thephotosensitive regions of the pixels, or by using a different colorfilter for the “small” pixels. As such, the present disclosure is easyand cheap to implement.

One aspect of the present disclosure involves an image sensor device.The image sensor device has a plurality of first pixels. Each of thefirst pixels is configured to have a first quantum efficiency. The imagesensor device has a plurality of second pixels, wherein each of thesecond pixels is configured to have a second quantum efficiency that islower than the first quantum efficiency.

Another aspect of the present disclosure involves an image sensordevice. The image sensor device has a first number of first pixelsdisposed in a substrate and a second number of second pixels disposed inthe substrate. The first number is substantially equal to the secondnumber. A light-blocking structure is disposed over the first pixels andthe second pixels. The light-blocking structure defines a plurality offirst openings and second openings through which light can pass. Thefirst openings are disposed over the first pixels. The second openingsare disposed over the second pixels. The second openings are smallerthan the first openings. A microcontroller is configured to turn ondifferent ones of the second pixels at different points in time.

Yet another aspect of the present disclosure involves a method ofoperating an image sensor device. The method includes turning on and offa plurality of first pixels at a first frequency. Each of the firstpixels senses light at a first quantum efficiency. The method includesturning on and off a plurality of second pixels at a second frequencythat is less than the first frequency. Each of the second pixels senseslight at a second quantum efficiency that is lower than the firstquantum efficiency.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure. For example, by implementing different thicknessesfor the bit line conductor and word line conductor, one can achievedifferent resistances for the conductors. However, other techniques tovary the resistances of the metal conductors may also be utilized aswell.

What is claimed is:
 1. An image sensor device, comprising: a pluralityof first pixels, wherein each of the first pixels is configured to havea first quantum efficiency; a plurality of second pixels, wherein eachof the second pixels has substantially identical dimensions as each ofthe first pixels and is configured to have a second quantum efficiencythat is lower than the first quantum efficiency; and a metal grid havinga plurality of openings that include a plurality of first openings and aplurality of second openings; wherein: each of the second pixelsincludes a first photo-sensitive portion and a second photo-sensitiveportion; and the first photo-sensitive portion is narrower than thesecond photo-sensitive portion and is located closer to the metal gridthan the second photo-sensitive portion.
 2. The image sensor device ofclaim 1, wherein a number of the first pixels is at least several timesof a number of the second pixels.
 3. The image sensor device of claim 1,wherein each of the first pixels has a different doping concentrationlevel than each of the second pixels.
 4. The image sensor device ofclaim 1, wherein: each of the first pixels includes a first colorfilter; and each of the second pixels includes a second color filterthat is more absorptive to light than the first color filter.
 5. Theimage sensor device of claim 4, wherein: the first color filter includesa red color filter, a blue color filter, or a green color filter; andthe second color filter includes a color filter other than a red colorfilter, a blue color filter, or a green color filter.
 6. The imagesensor device of claim 1, further comprising: a controller configured toselectively activate each of the first pixels and each of the secondpixels in a manner such that each of the second pixels is activated at asubstantially lower frequency than each of the first pixels.
 7. Theimage sensor device of claim 6, wherein a frequency in which each of thefirst pixels is activated is at least twice as high as a frequency inwhich each of the second pixels is activated.
 8. The image sensor deviceof claim 1, wherein: light is configured to pass through each of theplurality of openings; each of the first pixels is at least partiallyaligned with a respective first opening of the metal grid; each of thesecond pixels is at least partially aligned with a respective secondopening of the metal grid; and each of the second openings issubstantially smaller than each of the first openings.
 9. The imagesensor device of claim 1, further comprising: a plurality of isolationstructures disposed between the first pixels and second pixels, whereineach of the isolation structures is disposed beneath a respective metalportion of the metal grid.
 10. The image sensor device of claim 9,wherein at least two of the isolation structures separate one of thesecond pixels and one of the first pixels.
 11. An image sensor device,comprising: a plurality of first pixels disposed in a substrate, whereinthe first pixels are configured to sense radiation that enters thesubstrate from a back side of the substrate, wherein each of the firstpixels is configured to have a first quantum efficiency; a plurality ofsecond pixels configured to sense the radiation, wherein each of thesecond pixels is configured to have a second quantum efficiency that islower than the first quantum efficiency, wherein the first pixelsoutnumber the second pixels, wherein each of the second pixels includesa narrower first radiation-sensing portion and a wider secondradiation-sensing portion, and wherein the narrower firstradiation-sensing portion is disposed closer to the back side of thesubstrate than the wider second radiation-sensing portion; a metal gridhaving a plurality of first openings and a plurality of second openingseach configured to let radiation pass therethrough, wherein each of thefirst pixels is at least partially aligned with a respective one of thefirst openings, and wherein each of the second pixels is at leastpartially aligned with a respective one of the second openings; and acontroller configured to selectively activate the first pixels at ahigher frequency than the second pixels.
 12. The image sensor device ofclaim 11, wherein each of the first openings is wider than each of thesecond openings.
 13. The image sensor device of claim 12, wherein: eachof the first pixels is doped differently than each of the second pixels;or each of the first pixels is larger than each of the second pixels.14. The image sensor device of claim 11, wherein: each of the firstpixels includes a red color filter, a blue color filter, or a greencolor filter; and each of the second pixels includes a color filterother than a red color filter, a blue color filter, or a green colorfilter.
 15. The image sensor device of claim 11, wherein the controlleris configured to selectively activate the first pixels at a firstfrequency and activate the second pixels at a second frequency, whereinthe first frequency is at least twice as high as the second frequency.16. The image sensor device of claim 11, further comprising a pluralityof isolation structures disposed in the substrate and between adjacentones of the first pixels or second pixels, wherein a greater number ofthe isolation structures are disposed between an adjacent pair of firstpixel and second pixel than between an adjacent pair of first pixels.17. The image sensor device of claim 11, wherein the wider secondradiation-sensing portion of each of the second pixels is substantiallyas wide as each of the first pixels.
 18. An image sensor device,comprising: a first pixel, a second pixel, and a third pixel disposed ina substrate, wherein the first pixel, the second pixel, and the thirdpixel are each configured to sense, with a first quantum efficiency,light that enters a back surface of the substrate; a fourth pixeldisposed in the substrate, wherein the fourth pixel is configured tosense, with a second quantum efficiency, light that enters the backsurface of the substrate, wherein the fourth pixel has a narrowerradiation-sensing portion located closer to the back surface of thesubstrate and a wider radiation-sensing portion located farther awayfrom the back surface of the substrate; a first isolation structure anda second isolation structure disposed between the first pixel and thefourth pixel; a third isolation structure and a fourth isolationstructure disposed between the fourth pixel and the second pixel; and afifth isolation structure disposed between the second pixel and thethird pixel.
 19. The image sensor device of claim 18, further comprisinga metal grid having a plurality of openings that are each configured tolet the light pass through, wherein each of the openings is aligned witha respective one of the first, second, third, or fourth pixels.
 20. Theimage sensor device of claim 19, wherein each of the openings have asame size.